Flue gases from fossil fuel power plants contain sulfur primarily in the form of sulfur dioxide. Since sulfur dioxide is harmful to the environment, it is necessary to remove this pollutant from the flue gas before it passes into the atmosphere. Various processes have been proposed for the removal of sulfur dioxide from flue gases, including both wet and dry processes.
In a typical wet adsorption process, the flue gas is washed with an aqueous alkaline solution or slurry. Aqueous slurries of calcium oxide, calcium hydroxide or calcium carbonate have been used for removal of sulfur dioxide from flue gas. Other wet adsorption processes utilize an aqueous solution of soluble alkali such as sodium carbonate or sodium hydroxide to form sulfite and sulfate in solution. Although wet adsorption processes are effective in removing sulfur dioxide from flue gas, they suffer from the disadvantage of producing a liquid effluent containing a large amount of water relative to the sulfur dioxide adsorbed, so that the sulfur values cannot be economically and efficiently recovered from the aqueous solution. Another disadvantage of the wet scrubbing systems for flue gas desulfurization is the requirement for substantial quantities of make-up water for operation of the system. Suitable quality water is often available at the power plant only in limited quantities, which makes it necessary to reuse the water in the system.
In order to reduce the requirements for water, and to overcome the slurry disposal problems, dry sulfur dioxide removal processes have been proposed. In prior art dry processes, various methods are used to reduce the oxides by reaction with ammonia, hydrocarbons, carbon monoxide or hydrogen. These reducing agents are reacted with the flue gas in a precipitator to cause the sulfur removal to occur.
As indicated previously, the current state of the art relies heavily on discard-type scrubbing systems, i.e., treatment of the SO.sub.2 gas with lime or limestone in a wet scrubbing circuit, or alternatively, injection of alkali salts into the dry gas stream with subsequent particulate removal in a fabric-type bag house. The wet scrubbing techniques, if of the disposable reagent type, consume considerable amounts of energy and require large and expensive waste ponds for the storage of the spent solids.
There are also certain regenerable wet scrubbing techniques which utilize alkali salts, such as sodium hydroxide, as the make-up reagent. These systems remove the SO.sub.2 as a sulfite compound and are then thermally regenerated to remove the SO.sub.2 and prepare the solution for recycle. Again, these systems require the gas stream to be cooled to its saturation point which results in a considerable temperature reduction of the gas stream. Further, most of the regenerable systems require some sort of purge due to the oxidation of the sulfite solutions. Thus, the system is not totally closed, and the make-up alkali chemicals are generally considered expensive relative to the value of the SO.sub.2.